There has been an increased awareness of possible health hazards related to human ingestion of, or contact with suspect chemicals in polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC). Much effort has been devoted to removing vinyl chloride from PVC. Much effort has also been devoted to removing hydro-chloromethylenes and carbon tetrachloride (CCl.sub.4), very small quantities of each of which linger in CPVC even after determined steam stripping in a countercurrent continuous stripping column or "stripper".
Those skilled in the art are aware that the problem of stripping hydrochloromethylenes, such as dichloromethane and trichlormethane (chloroform, CHCl.sub.3), and CCl.sub.4 from CPVC are quite different from stripping vinyl chloride monomer from PVC. These chlorinated compounds are swelling agents which swell and wet macro-granular particles of PVC, hence are also referred to as swelling-wetting agents (hereafter "swelling agents" for brevity), which are used in a process for chlorinating PVC, to facilitate the substitution of chlorine in the PVC molecules. The term "macro-granular" refers to a granular resin in which essentially all of the particles are greater than about 10.mu. (microns), and preferably a preponderant portion above about 50.mu. in diameter.
The general size range of macrogranules is preferably from about 50.mu. to about 500.mu. in diameter, and typically is from about 100.mu. to about 200.mu. in diameter. Each macrogranule is made up of a multiplicity of primary particles in the size range from about 0.05.mu. to about 5.mu. in diameter, and more typically in the size range from about 0.5.mu. (5000A.) to about 2.mu. (20,000A). The surface area of the CPVC polymer in this particle size range is from about 0.5 m.sup.2 /g to about 80 m.sup.2 /g. The sizes and surface areas of the macrogranules, and those of the primary particles of which the macrogranules are constituted, are essentially the same whether the resin is PVC or CPVC. Also, like PVC primary particles, CPVC particles are porous. The porosity, or internal pore volume of suspension or mass-prepared CPVC is preferably in the range from about 0.05 cc/g to about 2 cc/g. Denser primary particles make stripping of the contaminants more difficult, as do larger primary particles, because the diffusion of the contaminants from within each particle to its surface controls stripping.
It will be evident that a more desirable solution to the problem of CHCl.sub.3 and CCl.sub.4 contaminants, is not to introduce either into the system in the first place. Recognizing that at present there is no viable alternative, these swelling agents are normally introduced into a high molecular weight PVC slurry after residual vinyl chloride monomer is stripped out. It is preferred to use only that amount of swelling agent as is necessary to effect the desired chlorination of PVC.
The process for preparing chlorinated polyvinyl chloride (CPVC) is set forth in more detail in U.S. Pat. Nos. 2,996,489; 3,100,762; and 3,167,535, the disclosures of which are incorporated by reference as if fully set forth herein. The CPVC is produced in a batch reaction in a chlorination reactor from which it is discharged as an acidic aqueous slurry containing from about 5 to about 25 percent by weight, based on solid resin, of CHCl.sub.3 and CCl.sub.4. Much, if not all of the CCl.sub.4 is produced by chlorination of CHCl.sub.3, one of the commonly used hydrochloromethylenes which are essential swelling agents. Typically however, some CCl.sub.4 is added to the chlorination reactor in a relatively small amount compared to the amount of CHCl.sub.3 added, the ratio by weight ranging from about 3 to about 50. The added CCl.sub.4 referred to is byproduct CCl.sub.4 carried over with recovery CHCl.sub.3. Typically the ratio of CHCl.sub.3 to byproduct CCl.sub.4 added to the chlorination reactor is in the range from about 5 to about 10. After the chlorination reaction is completed, the amount of CCl.sub.4 present is further enhanced due to chlorination of CHCl.sub.3 to byproduct CCl.sub.4. The amount of CCl.sub.4 present is increased in the range from about 20% to about 50% by weight over the initial amount of CCl.sub.4 introduced into the reactor.
This neutralized slurry is conventionally stripped with live steam in any suitable vessel such as a countercurrent stripping column, referred to as a primary steam stripper, which preferentially removes CHCl.sub.3 though CCl.sub.4 is also stripped. This primary stripping reduces the level of each contaminant, namely CHCl.sub.3 and CCl.sub.4, to less than about 5 percent by weight based on solid resin, and typically to less than 1.5 percent each by weight. Not unexpectedly, because of the manner in which the contaminants are combined or occluded within the porous primary CPVC particles in this low concentration, it is difficult to lower the contaminant concentration below about 1000 ppm even with a secondary steam stripper. Thus, despite additional, secondary steam stripping, commercially available CPVC typically contains well over 10 ppm each of CHCl.sub.3 and CCl.sub.4. For reasons not too clear at this time it appears that this difficulty is attributable to the presence of excess bound chlorine in essentially every primary particle. As used herein the term "excess chlorine" means chlorine in excess of that bound in the precursor by PVC.
Since CHCl.sub.3 and CCl.sub.4 are swelling agents which, under suitable conditions may be diffused out of CPVC, it would not seem too difficult, given enough time, to rid the CPVC of even traces of the contaminants utilizing a stripping agent such as steam which does not react with either the resin or the contaminants. But it is. It is difficult, time-consuming, and uneconomical, to strip essentially all CHCl.sub.3 with live steam. It is even more difficult, and for practical purposes, impossible to strip essentially all the CCl.sub.4 out of CPVC with live steam because the concentration of CCl.sub.4 appears to level off at about 10 ppm, based on solid resin.
A practical commercial process dictates that the process be economical. An analysis of the characteristics of the process and its variables dictates that the process be continuous and countercurrent. But there is nothing to suggest the choice of particular stripping agents which have an unexpectedly high stripping effectiveness combined with desirable compatibility within the system. By compatibility we refer to the ability of the stripping agent to discharge its function of stripping effectively without deleteriously affecting the physical or chemical properties of the CPVC slurry or the resin itself. Stripping agents in which the CPVC resin is slightly soluble tend to transform the CPVC slurry into a coherent solid mass, or a thick paste. Hence it is essential that a "non-solvent" stripping agent be used, that is, one which does not have an adverse solvent effect on CPVC. Stripping agents with only a slight proclivity to react with either the resin or the contaminants impart, at the very least, an undesirable color to the resin, and often exacerbate the problem of purification. Stripping agents which are effective at temperatures in excess of about 150.degree. C. are undesirable because the CPVC resin is sensitive to a temperature above about 150.degree. C. It is essential that the CPVC be stripped without being degraded to any extent which is unfavorably reflected in its processing characteristics. Stripping without degradation of the CPVC is a more important requirement than the time in which stripping may be accomplished.
Within the framework of the foregoing strictures, it is also essential from an economic point of view, that the CHCl.sub.3,CCl.sub.4 and stripping agent be recoverable for reuse preferably separately, in relatively pure form. Thus one skilled in the art is likely to avoid the use of a stripping agent which forms an azeotrope with either one or the other swelling agent, or both. Surprisingly however, it has been found that the most preferred stripping agents, namely methanol and hexane are azeotrope formers. Methanol forms an azeotrope with CHCl.sub.3 and CCl.sub.4 ; hexane forms an azeotrope with CHCl.sub.3.